1. Field of the Invention
The present invention relates to a piezoelectric diaphragm for use in a piezoelectric receiver, a piezoelectric sounder, or other suitable apparatus, and also to a piezoelectric electroacoustic transducer including such a piezoelectric diaphragm.
2. Description of the Related Art
Piezoelectric electroacoustic transducers are widely used as piezoelectric sounders, piezoelectric receivers, or other elements in electronic devices, home electronic appliances, portable telephones, or apparatuses. In such piezoelectric electroacoustic transducers, it has been proposed in Japanese Unexamined Patent Application Publication No. 2001-95094 to use a bimorph piezoelectric diaphragm made of multilayer piezoelectric ceramic.
The piezoelectric diaphragm disclosed in Japanese Unexamined Patent Application Publication No. 2001-95094 is in the form of a multilayer ceramic body produced by laminating two or three piezoelectric ceramic layers. Principal-surface electrodes are disposed on upper and lower principal surfaces of the multilayer ceramic body, and an internal electrode is disposed between adjacent ceramic layers. All ceramic layers are polarized in the same transverse direction. The multilayer ceramic body vibrates in the flexure mode in response to an AC signal applied between the internal electrode and the upper and lower principal-surface electrodes. In this piezoelectric diaphragm including two ceramic layers that are disposed one on top of the other, the two ceramic layers function as vibration layers that vibrate in opposite directions. Therefore, this type of piezoelectric diaphragm can be displaced to a greater degree in vibration than the unimorph type of piezoelectric diaphragm, and thus a greater sound pressure can be achieved.
However, because this piezoelectric diaphragm is made of only ceramic layers, the piezoelectric diaphragm is easily broken when a mechanical shock such as a drop impact is applied to it. To solve this problem, Japanese Unexamined Patent Application Publication No. 2002-10393 discloses a piezoelectric diaphragm in which upper and lower surfaces of a multilayer ceramic body are substantially entirely covered with a thin resin layer to reinforce the multilayer ceramic body. This results in a great improvement in resistance against drop impacts. Besides, the resin layers do not influence the flexure vibration of the multilayer ceramic body, and thus, the resin layers do not cause significant changes in the sound pressure and the resonant frequency. The reinforcement of the multilayer ceramic body with the resin layers allows a reduction in thickness of the ceramic layers, which allows a further increase in sound pressure.
FIGS. 8 to 10 show an example of such a piezoelectric diaphragm.
As shown in those figures, the piezoelectric diaphragm 40 includes a multilayer ceramic body 41 formed by laminating two layers 41a and 41b of piezoelectric ceramic such as PZT. Principal-surface electrodes 42 and 43 are formed on the upper and lower principal surfaces, respectively, of the multilayer ceramic body 41. An internal electrode 44 is disposed between ceramic layers 41a and 41b. The two ceramic layers 41a and 41b are polarized in the same thickness direction as represented by arrows P. The principal-surface electrode 42 on the upper principal surface and the principal-surface electrode 43 on the lower principal surface each extend from one side of the multilayer ceramic body 41 to a location close to the other side. In contrast, the internal electrode 44 extends from the other side of the multilayer ceramic body 41 to a location close to the one side. The upper and lower principal surfaces of the multilayer ceramic body 41 are covered with resin layers 45 and 46, respectively.
A side surface electrode 47 is provided on one side surface of the multilayer ceramic body 41 such that the side surface electrode 47 is electrically connected to the principal-surface electrodes 42 and 43. On the opposite side surface of the multilayer ceramic body 41, a side surface electrode 48 is arranged such that the side surface electrode 48 is electrically connected to the internal electrode 44. The upper and lower end portions of the side surface electrode 48 are electrically connected to lead electrodes 49 provided on the side edges of the upper and lower surfaces of the multilayer ceramic body 41. Cutouts 45a and 46a are formed in the respective resin layer 45 and 46 such that the principal-surface electrodes 42 and 43 on the upper and lower principal surfaces are partially exposed in the cutouts 45a and 46a, and cutouts 45b and 46b are formed in the respective resin layer 45 and 46 such that the lead electrodes 49 are partially exposed in the cutouts 45b and 46b. The piezoelectric diaphragm 40 is put in a case with terminals, and the electrodes exposed in the cutouts 45a and 45b formed in the resin layer 45 on the upper principal surface are connected, via conductive adhesives, to respective terminals exposed inside the case thereby achieving electrical connections between the piezoelectric diaphragm 40 and the terminals.
In the piezoelectric diaphragm 40 constructed in the above-described manner, the cutouts 45a, 46a, 45b, and 46b are formed in the resin layers 45 and 46 such that the cutouts 45a and 45b are located at the center of one side edge of the respective resin layers 45 and 46 and thus, they oppose each other in a direction across thickness of the resin layers 45 and 46, while the cutouts 45b and 46b are located at the center of the opposite side edge of the respective resin layers 45 and 46 and thus, they oppose each other in the direction across thickness of the resin layers 45 and 46. The cutouts 45a and 46a are formed in the upper and lower resin layers 45 and 46 such that the principal-surface electrodes 42 and 43 on the upper and lower principal surfaces are partially exposed in the cutouts 45a and 46a for the following reasons. The side surface electrodes 47 and 48 are formed after the resin layers 45 and 46 are formed on the upper and lower principal surfaces of the multilayer ceramic body 41. However, because the principal-surface electrodes 42 and 43 are formed of thin films, very small areas of the principal-surface electrodes 42 and 43 are exposed between the resin layers 45 and 46 and the ceramic layer 41a and 41b. Therefore, if the side surface electrode 47 is simply formed on the side surface of the piezoelectric diaphragm 40, reliable electrical connections between the side surface electrode 47 and the principal-surface electrodes 42 and 43 cannot be achieved. To avoid this problem, cutouts 45a and 46a are formed in sides, along the side surface electrode 47, of the upper and lower resin layer 45 and 46, and the side surface electrode 47 is formed such that a portion of the side surface electrode 47 extends on the surface of the principal-surface electrodes 42 and 43 thereby achieving reliable electrical connections. Although in this example, the cutouts 45b and 46b for exposing the lead electrode 49 therein are formed in both upper and lower resin layers 45 and 46 in order to achieve symmetry in structure, the cutout 46b in the lower resin layer 46 is not necessarily needed.
However, because cutouts 45a, 46a, 45b, and 46b are formed at opposing locations in the upper and lower resin layers 45 and 46, if a mechanical shock is applied to the piezoelectric diaphragm 40, for example, due to dropping, cracks are often created in portions of the multilayer ceramic body 41 exposed in the cutouts. This problem becomes more serious, in particular, when the thickness of the multilayer ceramic body 41 is reduced. In FIG. 10, broken lines CR represent locations where cracks are produced. A cause of cracks is stress produced by contraction of the conductive adhesive which occurs when the adhesive is cured, and the stress is applied to the electrodes exposed in the cutouts formed in the resin layers 45 and 46. Another cause of cracks is that when a drop impact is applied, impact force to the case is intensely applied via the conductive adhesive to the portions of the electrodes exposed via the cutouts formed in the resin layers 45 and 46. Still another reason for the cracks is that the provision of the cutouts 45a, 46a, 45b, and 46b results in a reduction in the mechanical strength of the multilayer ceramic body 41.